Added extensibility points

This commit is refactor only (there is no behavior changes and no new features added). We've added some extensibility points to have a possibility to override some methods in our classes generated by http://EasyWSDL.com generator.
Additionally we started to work on MTOM transfer (SOAP with attachments) and those extensibility points allow us inject a new features into ksoap2 library.

We've already tested this commit and all our tests still works good (so existing behavior of ksoap2 hasn't been changed)